Preheat the oven to 350 degrees F. Line a 9-inch baking pan with parchment paper long enough to extend over the sides of the pan.
In a medium-sized bowl, whisk together the flour, sugar, baking powder, and salt.
Add the cubes of butter and incorporate the mixture into the flour using a pastry cutter. The butter should be finely dispersed throughout the flour mixture, and the texture should resemble wet sand.
Transfer 2 ½ cups of the mixture into the base of the baking pan.
Using the bottom of a measuring cup or other flat-bottomed jar, press down firmly to create a solid, dense base layer. Set it aside.
In a large bowl, combine the peaches, sugar, flour, cinnamon, and salt.
Add the lemon juice and vanilla extract, stirring until well combined.
Transfer the mixture to the shortbread base, and top with the remaining crumble mixture.
Bake for approximately 50-55 minutes, or until the peach filling is bubbling at the sides of the pan and the crumble is golden.
Remove the baking pan from the oven and transfer it to a wire cooling rack to cool for 15-20 minutes.
Using the parchment paper as ‘handles’, gently remove the peach bars from the pan.
Place the peach bars on a cutting board and let them cool for another 30 minutes before cutting them into squares.